As a system which manages the position where an article is present, there is available a system in which a barcode is put on each of shelves in a warehouse and a barcode is also put on each of articles stored in the warehouse, so that when an article is stored on a shelf, information of the barcode put on the shelf and information of the barcode put on the article are read by a barcode reader (see, e.g., [1]). In this system, a terminal device with the barcode reader sends the pieces of information of both the barcodes to a management device via wireless communication. The management device stores the pieces of information of both the barcodes sent from the terminal device.
There has also been proposed a system which uses a wireless IC tag (to be referred to as an RFID tag hereinafter) instead of a barcode (see, e.g., [2]). More specifically, an RFID tag as an asset tag is put on an article (an asset), and an RFID tag as a place tag is affixed to a pole, wall, or the like. Information of the asset tag and of the place tag are read by a PDA with an RFID tag reader, and the PDA sends the pieces of information of both the RFID tags to a server over a wireless LAN. The server stores the pieces of information of both the RFID tags sent from the PDA.
As described above, the place in a predetermined region where an article is present can be managed on the basis of information of a tag put on the article and positional information (a barcode put on a shelf or a place tag affixed to a pole, wall, or the like) indicating the place where the article is present. In addition to the above-described systems, there is available a system using GPS-based positional information (see, e.g., [2]). In this system, an RFID tag is put on a container (an article). When a forklift equipped with a GPS receiver places the container in a predetermined position, it reads information of the RFID tag put on the container and sends the information of the RFID tag and positional information acquired by the GPS to a server over a wireless LAN. The server stores the sent pieces of information.
Examples of an RFID tag include an active tag with an internal battery in addition to a passive tag without an internal power supply. There has also been proposed a system in which an active tag is put on an article or the like, antennas for receiving information transmitted from an active tag are installed all over the place, and the position of an article or the like is detected on the basis of the location of an antenna that has received information (see, e.g., FIG. 3 of [3]).
As a system for detecting the position of an article such as a wireless LAN terminal, there has also been proposed a system which uses a wireless LAN base station (see, e.g., [4]). In this system, the position of an article such as a wireless LAN terminal is determined by trilateration based on differences in the received strength and propagation delay time of a radio wave which each of three or more base stations receives from a wireless LAN terminal.
Document [3] describes a system in which tag readers are installed all over the place, and the position of an article is determined by trilateration based on a difference in the received strength of a radio wave which each of three of the tag readers receives from an active tag put on the article (see, e.g., FIG. 2 of [3]).